1. Field of the Invention
The present invention generally relates to image sensors and more particularly to high resolution image sensors.
2. Background Description
Digital cameras have largely replaced film based analog cameras, at least for amateur photography. A typical digital camera image sensor is an array of picture cells (pixels), each sensing a small fragment of the light for an entire image. Generally, the higher the number of pixels, the better the resulting images (pictures) and the larger an image may be viewed before becoming pixilated. Thus, the number of pixels is a primary measure of the image resolution, and directly affects the sharpness and crispness of the resulting images. Early digital cameras included bucket brigade sensors with Charge Coupled Devices (CCDs) for pixel sensors. Integration, power, and frame rate considerations have driven the industry to convert from CCDs to image sensors that are based on more standard CMOS logic semiconductor processes.
A typical CMOS image sensor array is, simply, an array of photodiodes with connected CMOS support and sensor circuits. Light striking each photodiode changes its current-voltage (I/V) characteristic and the CMOS support senses characteristic changes in each diode. A color pixel sensing red, green or blue is just an appropriately filtered diode, with a red, green or blue filter to block all light outside of the particular bandwidth, i.e., red, green or blue. CMOS image sensors have allowed pixel density to increase well above 4 MegaPixels (4 MP), even as typical digital cameras have gotten more and more compact, e.g., some are even embedded in cell phones.
Unfortunately, as pixel areas have shrunk to improve density, the amount of light to each sensor, which is directly related to photodiode surface area, has also diminished. So for CMOS sensors in particular, as pixel density has increased, the diminished sensor light (sensor signal) may be lost in chip noise, especially in low light conditions. For example, compact cameras with 6-8MP resolution, have been criticized for noisy results, even for pictures taken at sensitivity settings as low as 100 (ISO 100). Thus, this noise is an impediment to increasing pixel density for CMOS imaging sensors, while maintaining a small chip footprint for compact applications.
Thus, there is a need for improved CMOS imaging sensor sensitivity, especially under low light and reduce CMOS sensor low light noise.